It is well known that many naturally occurring clays may be acid treated to enhance their ability to decolorize oils. One of the most common clays used in this field is bentonite clays, particularly the non-water swelling bentonite clays such as calcium and magnesium bentonites. These acid activated bentonite clays are used for decolorizing or bleaching of various oils such as mineral oils, vegetable oils, various fractions from petroleum, especially lubricating oils, and the refining of used lubricating oils, and purification of melted animal fats and beeswax.
In the early 1900's, it was discovered that the bleaching power of clays such as bentonite could be enhanced far better than the bleaching power of the best crude clays in use at that time provided that the clay was first treated with a mineral acid such as hydrochloric or sulphuric acid and washed substantially free of dissolved salts and residual acid. The general process of acid activation of clays, as used today in industry, includes grinding the clay for uniform activation and dispersion in water; slurrying the ground clay in water and then adding acid to the clay slurry in an attempt to achieve a substantially uniformly activated clay. Usually about 6 to 8 hours of acid digestion is necessary to properly acid activate the clay. The digested slurry then is washed substantially free of dissolved salts such as iron and aluminum sulfates or chlorides, depending upon the particular mineral acid used. The acid activated clay then is dewatered, such as by filtration, and the resulting filter cake is dried and subsequently ground to product specifications.
One of the main disadvantages of the presently used commercial process for acid activation of clays is that the filtration and washing steps proceed very slowly, such as in a filter press. Presently existing commercial processes using a filter press for filtration and washing result in a filter cake having only 35 to 40% by weight solids. Accordingly, the energy requirements for the removal of most of this 60 to 65% weight by water in the drying operation is excessive and uneconomical.
The following patents describe various processes for treatment of clay to enhance the filtering characteristics of the clay in the removal of colorizing compounds: U.S. Pat. Nos. 1,408,655; 1,524,843; 1,408,656; 1,402,112; 1,544,210; 1,731,702; and 1,739,734.